I have tried opening MR2 reports (Impromptu client) in MR1 with success.

I think 6.0.xxx or 7.0.xxx are cross compatible across their minor (sub)
versions.

However, the server components (IWR) of above versions may give
incompatibility, as Cognos has discouraged customers on having different
versions of Server Administrator and Application Servers.
I havent tested this part, though.

Has anyone had any problems with the Server Administrator and application
servers having version differences ?

If I am currently running PPES 6.0 and PP Client/Transformer 5.21, should I
upgrade all or only some components to 6.61?

Solution:

If you are going to consider upgrading from 5.21 and 6.0 to 6.61, it is
recommended that serious consideration be put into upgrading the cube
building process [Transformer] and the cube deployment application
[Enterprise Server] to the same version and edition.

Upgrade one and not the other will result in incompatibilities. PowerPlay
Enterprise Server and PowerPlay Transformer all make use of a PowerPlay Data
Server to read and write cubes. For overall compatibility and consistency,
it is advisable to upgrade all Cognos PowerPlay components to make use of
the same PowerPlay Data Server.

Should you choose to only upgrade one component, the first to upgrade should
be PowerPlay Enterprise Server. The second would be to upgrade PowerPlay
Transformation Server Windows or Unix which includes PowerPlay Client and
Transformer. This way you will not have cubes that PowerPlay cannot
read/understand. If you do it the other way, you may build cubes using the
newest features and try to access these cubes using an older PowerPlay
Enterprise Server that cannot read these cubes with the latest features.

To successfully migrate the models and cubes with Transformer will require
the following steps:

1. Install 6.61 and 5.21 Transformation Server Windows on a pc. They can
both co-exist for this purpose.
2. Let's create two folders: 521Archives and 661Work
3. Each 5.21 pyg model should be opened and saved as an mdl file.
4. Save a copy of each model to two locations: one to the 521Archives for
archival purposes and the second to 661Work. This creates a situation that
you can go back to 5.21 if your corporation so decides or wishes to have an
archive of prior models.
5. Once the models have been saved as mdl files, we can then proceed to
rebuilding the models in 6.61 using the models in 661Work.
6. Using 6.61, open each model and save as pyi if your corporate policy is
to have both mdl and pyi.
6A. Optional: associate the user classes and security to the models if
necessary.
7. Rebuild the cubes.
8. Deploy the cubes on PowerPlay Enterprise Server.
9. Once the models have been migrated, you may uninstall 5.21. As 5.21 does
not include an automated uninstall to do this, delete the entire directory
structure.

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